A system of typical vapor compression refrigerating cycle is composed as shown schematically in FIG. 13. The cycle is shown in FIG. 14 as a T-S diagram with temperature as the ordinate and entropy as the abscissa, in which the cycle operates the process a-b′-c-d″-a.
That is, saturated vapor of a refrigerant at point a is compressed adiabatically to point b′ by a compressor 02, then cooled from point b to point a under constant pressure in a condenser 04 to be condensed to saturated liquid at point c while heat quantity of Q1 being deprived of the refrigerant. The saturated liquid is expanded through an expansion means (expansion valve) 06 to be decreased in pressure from P2 to P1 through an isenthalpic expansion process c-d″. The refrigerant is in a state of wet vapor at point d″, i.e. a mixture of saturated liquid of state point c and saturated vapor of state point a. The saturated liquid in the wet vapor evaporates in an evaporator 08 under pressure P1 and absorbs heat quantity of Q2 from specified substance, thus refrigeration is effected.
A vapor compression refrigerating cycle like this can be considered as a cycle based on the reversed Carnot cycle.
FIG. 16 shows the Carnot cycle on a T-S diagram. When the Carnot cycle is operated in a reversed direction, i.e. in a direction shown by arrows to operate the process of a-b-c-d, a refrigerating cycle is effected. In FIG. 16, the process a-b is adiabatic compression, process b-c is isothermal compression, process c-d is adiabatic expansion, and process d-a is isothermal expansion.
Applying the reversed Carnot cycle of FIG. 16 to the T-S diagram of the vapor compression refrigerating cycle of FIG. 14, each of processes a-b, b-c, c-d, and d-a in FIG. 14 can be considered to correspond to each of processes represented by the same symbols in the reversed Carnot cycle of FIG. 16. That means that the vapor compression refrigerating cycle can be considered as a cycle for operating the below the line of saturation (saturated liquid line l-l′ and dry saturated vapor line m-m′, both lines coincide at the critical point not shown in the drawing). In FIG. 14, a-b is adiabatic compression process, b-g is isothermal compression process, g-c is isothermal condensation process, c-d is adiabatic expansion process, and d-a is isothermal evaporation process.
The feature of the reversed Carnot cycle a-b-c-d-a in FIG. 14 can be considered schematically that the isothermal compression process b-c and isothermal expansion process d-a of the Carnot cycle are replaced by the condensation process g-c and evaporation process d-a by allowing a large part of the cycle to operate below the line of saturation with only the process b-g belonging to a part of isothermal compression process of the Carnot cycle.
As isothermal compression process is difficult to realize, the process b-g outside the dry saturated vapor line is replaced by the adiabatic compression process b-b′ and isobaric cooling process b′-g in the actual vapor compression refrigerating cycle.
Also, as isentropic expansion process c-d is difficult to realize in adiabatic expansion of 2-phase refrigerant consisting of vapor and liquid refrigerant in the actual vapor compression refrigerating cycle, isenthalpic expansion process c-d″ is substituted for the isentropic expansion process c-d by use of an expansion valve in the actual vapor compression refrigerating cycle.
FIG. 15 is a P-H diagram (pressure-enthalpy diagram) for T-S diagram of FIG. 14.
As has been explained, the typical vapor compression refrigerating cycle can be considered a practical cycle based on the reversed Carnot cycle.
More specifically, as mentioned above, the feature of the vapor compression refrigerating cycle can be considered a cycle intended for putting the Carnot cycle to practical use, in which a large part of the isothermal compression process of the reversed Carnot cycle a-b-c-d-a of FIG. 16 is replaced by the isothermal condensation process g-c by utilizing the characteristic of wet vapor below the line of saturation, the remainder, i.e. the process b-g, is replaced by the adiabatic compression process b-b′ and isobaric process b′-g, further the isentropic expansion process is replaced by the isenthalpic expansion process which is realized by use of an expansion valve, and the isothermal expansion process by the isothermal evaporation process.
By the way, there is known the Stirling cycle and Ericsson cycle as reversible cycles in addition to the Carnot cycle.
FIG. 17 is a T-S diagram of the reversed Stirling cycle, in which process a-b is isometric heat absorption, process b-c is isothermal compression, process c-d is isometric heat dissipation, and process d-a is isothermal expansion. The amount of heat absorbed in the isometric heat absorption process a-b is equal to that dissipated in the isometric heat dissipation process c-d, the heat exchange being done through the intermediary of a regenerating heat exchanger.
FIG. 18 is a T-S diagram of the reversed Ericsson cycle, in which process a-b is isobaric heat absorption, process b-c is isothermal compression, process c-d is isobaric heat dissipation, and process d-a is isothermal expansion. The amount of heat absorbed in the isobaric heat absorption process a-b is equal to that dissipated in the isobaric heat dissipation process c-d, the heat exchange being done through the intermediary of a regenerating heat exchanger.
There are many proposals of refrigerators using the typical vapor compression refrigerating cycle such as disclosed for example in Japanese Laid-Open Patent Application No. 2004-108617, No. 2002-156161. In Japanese Laid-Open Patent Application No. 55-60158 is recited the theoretical coefficient of performance when considering the vapor compression refrigerating cycle as the reversed Carnot cycle (see page 2, the middle part of upper right column of the official gazette), thus it is known to evaluate the vapor compression refrigerating cycle presuming the reversed Carnot cycle of the vapor compression refrigerating cycle.